Authentication Mechanisms

Plaintext authentication

The simplest authentication mechanism is PLAIN. The client simply sends the password unencrypted to Dovecot. All clients support the PLAIN mechanism, but obviously there's the problem that anyone listening on the network can steal the password. For that reason (and some others) other mechanisms were implemented.

Today however many people use SSL/TLS, and there's no problem with sending unencrypted password inside SSL secured connections. So if you're using SSL, you probably don't need to bother worrying about anything else than the PLAIN mechanism.

Another plaintext mechanism is LOGIN. It's typically used only by SMTP servers to let Outlook clients perform SMTP authentication. Note that LOGIN mechanism is not the same as IMAP's LOGIN command. The LOGIN command is internally handled using PLAIN mechanism.

Non-plaintext authentication

Non-plaintext mechanisms have been designed to be safe to use even without SSL/TLS encryption. Because of how they have been designed, they require access to the plaintext password or their own special hashed version of it. This means that it's impossible to use non-plaintext mechanisms with commonly used DES or MD5 password hashes.

If you want to use more than one non-plaintext mechanism, the passwords must be stored as plaintext so that Dovecot is able to generate the required special hashes for all the different mechanisms. If you want to use only one non-plaintext mechanism, you can store the passwords using the mechanism's own password scheme.

With success/failure password databases (e.g. PAM) it's not possible to use non-plaintext mechanisms at all, because they only support verifying a known plaintext password.

Dovecot supports the following non-plaintext mechanisms:

CRAM-MD5: Protects the password in transit against eavesdroppers. Somewhat good support in clients.